In recent years, with the explosive popularization of notebook-size personal computers and personal digital assistants, demands have been prompted for rechargeable small-size, light-weight, high-capacity, high energy density, and highly reliable secondary batteries. Further, in automobile industry, there have been great expectations to reduce carbon dioxide emissions by the introduction of electric vehicles (EV) and hybrid electric vehicles (HEV), and the development of secondary batteries for driving motors has been also actively carried out, which holds the key to practical use of the vehicles. In particular, lithium ion secondary batteries considered to have the highest theoretical energy among batteries have been attracting attention, and rapid development of the batteries has been now advanced rapidly.
The lithium ion secondary battery is typically configured to have a positive electrode obtained by applying a positive electrode active material such as composite oxide including lithium onto a current collector such as aluminum with the use of a binder and a negative electrode obtained by applying a negative electrode active material capable of storing and releasing lithium ions onto a current collector such as copper with the use of a binder, and to have the positive electrode and negative electrode connected and hermetically sealed with a separator and an electrolyte layer interposed therebetween.
For increasing the capacity and energy density of the lithium ion secondary battery, the use of metals such as silicon, tin, and aluminum forming alloys with lithium ions, as well as oxides thereof, in addition to graphite materials widely used conventionally has been considered for the negative electrode active material. In particular, negative electrode active materials containing silicon are high in theoretical capacity per unit mass, and expected to be significantly improved in energy density, and both silicon and silicon oxides have thus been actively considered.
On the other hand, the negative electrode active materials containing silicon are known to have the problems of high volume expansion with the storage of lithium ions, and the electrode conductivity decreased, that is, the capacity retention decreased with the expansion and contraction of the electrode when the absorption and desorption of lithium ions are repeated, and there is strong demand for solutions to solve the problems.
As an approach to solving the problems, for example, the use of, as an active material, a silicon oxide SiOx (1≤x<1.6) powder with the surface coated with a conductive film by a chemical vapor deposition treatment has been proposed (Patent Document 1). According to this method, it is proposed that the formation of the conductive film on the powder surface by the chemical vapor deposition treatment ensures electrode conductivity, while the failure to improve the conductivity decreased by internal collapse of the active material due to expansion and contraction, as well as the initial discharge capacity significantly decreased with respect to the initial charge capacity, that is, the decreased initial efficiency still exist as problems.
Against the problems, it is disclosed that the use of, as an active material, particles structured to have silicon nanoparticles dispersed in silicon oxide reduces the sizes of the silicon particles dispersed in silicon oxide to prevent the internal collapse of the active material due to expansion and contraction and improve the capacity retention (Patent Document 2), or that etching under an acidic atmosphere reduces the silicon oxide constituent to improve the initial efficiency (Patent Document 3).
Furthermore, examples related to the improvement in initial efficiency and focused on the contained oxygen amount of active material particles include Patent Document 4. The invention in question has a feature that an active material layer including active material particles containing silicon and/or a silicon alloy, and a binder is placed on the surface of a current collector composed of conductive metal foil, and then subjected to sintering under a non-oxidizing atmosphere so that the oxygen content of the active material particles is 0.5 weight % or less.
In addition, Patent Document 5 discloses, as a method for obtaining silicon particles containing no oxygen, a method of obtaining a spherical silicon powder by applying a reduction treatment to a spherical silica powder of 1 to 100 nm in average particle size obtained by applying flame hydrolysis to a gas mixture of silicon chloride or silane, oxygen, and hydrogen mixed.